Several methods have been used to achieve the motions, torques, and forces for the cyclic operations required to form individual finished articles in a manufacturing apparatus made up of multiple operational units performing successive operations on web materials. For example, in the manufacture of disposable absorbent articles, a common approach is to first form a composite product web and then individual finished articles. A main web is typically advanced in a machine direction and secondary webs and/or discrete components are applied to the main web to form a composite web, from which the discrete finished products are eventually severed. For the formation and application of discrete components, a slower secondary web is often made to slip or slide on a surface moving at a speed close to that of the composite product web and each discrete component is detached and then accelerated due to the movement of the surface. By phasing the detachment, the components can be approximately phased relative to the pre-determined portions of the composite product web corresponding to the individual products to be formed from the product web. However, the equipment for slipping and cutting the secondary web is typically suitable for only a limited range of pitch length of the finished product. Outside this range, the mismatch between the speed of the surface on which the secondary web slips and the speed of either the secondary web or the composite product web is too great for reliable operation. Therefore, when it is desired to make a product with a different pitch length outside the range of suitability of a particular piece of equipment, often the entire operational unit must be replaced with one designed for the different product pitch length. The synchronization of the discrete component is typically performed via an external registration system or by means of a system in which an external automatic or operator-assisted feedback control system is used to adjust the phase of the operational unit or of the main web to achieve proper placement.
For the cutting and removal of portions of materials, and for the severance of individual finished products, the operational units are often similarly limited in the range of product pitch lengths for which they are suitable. Although cuts can be made with moderately mismatched web and cutter speeds, the magnitude of tolerable mismatch is restricted by the thickness of the portion of the composite product web being cut, the thickness of adjacent portions, the fragility of the material being cut, and other factors. Therefore, for product pitch changes, it is common to replace rolls and operational units performing these process functions. Synchronization of these cutting operations is also typically achieved by a phasing action via an automatic or operator-assisted control system.
In many manufacturing apparatuses, the operational units are driven from one or more mechanical line shafts. This approach makes it possible to establish and maintain the synchronization of several operational units. However, this approach has the disadvantages of relative inflexibility. For instance, changes in the product pitch length, size, or design, often require changes in ratios that, in turn, require changes in the mechanical power transmission equipment, such as the substitution of several gears or pulleys. Also, since power transmission equipment that is designed for relatively easy changes of the above type often has numerous components requiring relatively precise setup, the costs of the setup, adjustment, maintenance, and replacement of such equipment may be high.
Servo motors and programmable controllers have been applied in attempts to address the above described interrelated process and equipment issues. However, the uses of servos on these types of manufacturing lines have typically been limited to web-based operational units achieving synchronization via registration systems with line shaft speed generation reference signals being used to provide the velocity references for one or more servo motors. In some applications, position based synchronization of operational units relative to the product position has been achieved via timing-based systems tied with external sensors. For example, servo technology has been used to accelerate and decelerate rolls in time-based control schemes and to dynamically adjust registration with respect to sensed marks on the product web.
The previous approaches to using servo technology fail to fully address the need for the synchronization of cyclic operations over a wide range of product pitch lengths, sizes, and designs, without the need for duplicate operational units for different products and with the flexibility of a fully servo driven manufacturing apparatus not requiring a line shaft.
Thus, the need exists for a practical method for the synchronization of a plurality of servo actuators, and in particular a method that provides the necessary coordination of the servo actuation to X, Y and Z coordinates of a composite web from which individual discrete finished products are being formed.